1. Field of the Disclosure
This invention is related to the control of switched-mode power supplies. Specifically, it is related to low-cost power supplies with regulated isolated outputs that must meet standards for maximum power consumption when the output has no load, and yet must keep the output within specified limits when a load is suddenly applied.
2. Background
Low-cost solutions to regulate an isolated output voltage of a switching power supply typically rely on the magnetic coupling between isolated windings of an energy transfer element to provide information about the output to a control circuit. The control circuit typically receives a signal representative of the output voltage immediately after a switching event that delivers energy to the output. The signal is typically received from a primary-referenced winding of an energy transfer element instead of from an optocoupler. This type of control is often referred to as “primary-side control” or control using primary-side feedback.
Although these solutions eliminate the cost and the power consumed by an optocoupler, they cannot sense the output voltage in the absence of switching. A problem arises when the load on the output of the power supply approaches zero. The power supply must provide the specified regulated output voltage but almost no power. Under such conditions, the power lost in the operation of the power supply itself is a significant part of the total power consumed. Requirements to limit the consumption of power by the power supply under conditions of near zero external loading discourage the use of a dummy internal load in a power supply. A dummy internal load, sometimes called a pre-load, can be useful in a power supply to help provide overvoltage protection, improve regulation between multiple outputs, and prevent the switching frequency from going below a minimum value. In particular, a dummy internal load is a small permanent minimum load inside a power supply. However, a penalty for using a dummy internal load is that the power supply becomes less efficient because the dummy load dissipates power that is not measured as output power. Also, the controller has to switch more often to power the additional internal load, which results in the power supply consuming additional power even though there is no load connected to the power supply's output. To avoid these drawbacks, it is useful to increase the time between switching events under no-load conditions to reduce the losses inherent in switching. However, under such conditions, the controller is unable to sense the output voltage during the relatively long intervals between switching events.
When a substantial load is suddenly applied to the output during one of the relatively long intervals between switching events, the output voltage can easily fall outside the specified limits of regulation before the controller is able to respond to the condition. A typical remedy for such a condition is the addition of costly bulk capacitance to the output to provide the energy required by a load that could be applied during the time when the controller cannot sense the output.